One probably has to cut this film some slack to a certain degree. Because it had a very tough act to follow. How exactly were they supposed to top Marvel's The Avengers? Personally, my answer would be that you can't and you should just do what you want and not worry too much about competition, because it's too late for that. But instead they tried. And tried so hard--and yet they missed the mark. So badly.
To clarify: this isn't a bad movie at all, but given the level of expectations and the perceived scope, it's quite underwhelming. One arguably could have expected this, given some of the issues that the other DC Extended Universe films so far had. But it's still kind of depressing no matter what when a supposedly monumental event movie like this comes up, and it falls as short as it does.
This movie's plot actually is very similar to The Avengers. A bad guy named Steppenwolf invades with his army of Parademons, who is looking for the "Mother Boxes" -- three cubes of insane and rather vague power. (So they're basically the Tesseract, but there's only three of them.) So he can do what? Destroy the world. In the wake of the death of Superman, Bruce Wayne/Batman is forced to put together a team (how many times have you heard *that* in the last five years?), which includes Wonder Woman, The Flash, Aquaman, and Cyborg.
Interestingly enough, this movie's kind of a different beast from its predecessors. It's far less convoluted, for one thing. But it also kind of ditches some of the strengths of its predecessors. One thing that did set apart the DC movies was the darker tone. That's mostly gone, in favor of a much more inconsistent tone. Sometimes it is still kind of dark. But sometimes it's also much lighter. And sometimes... I don't think even it knows what it is. Also, one thing that these films had been good at in the past was a good climactic fight. Here? Well, it doesn't suck; but it's not nearly as well-done either. That's probably in part because because of the extremely overdone CGI, and also in part because of the shockingly generic villain that is Steppenwolf.
As for the heroes themselves? Well, depends upon which one we're talking about. Batman isn't really given as much to do this time; kind of understandable since he can't fight against Steppenwolf, but he even struggles with Parademons. He's just kind of there most of the time. Wonder Woman remains one of the better things about the franchise; an early scene where she dispatches some terrorists is one of the better scenes in the movie. The Flash is pretty cool, and also provides some comic relief. Aquaman is better than you'd expect... though that's not really saying much, and I'm still very skeptical of his upcoming movie. Cyborg has some interesting powers, but he suffers the most from another glaring issue: lack of character development. That's mostly because this movie is too short for a movie of this scope, but Cyborg suffers the issue the worst.
And speaking of being too short... that's probably the biggest problem altogether. People complained Batman vs Superman was too long, and maybe they were kind of right. But this is a movie that you need to be longer than just 120 minutes, in part for the sake of character development--which is all the more important when half the characters haven't been introduced before. And really, this whole thing just feels so rushed. The bad guy attacks. The team gets together. One massive event happens about two-thirds of the way through which creates one of the more interesting scenes of the movie. Then the team is established. Then the final battle happens. Then... well, that's it.
For all of the elements that feel either all too familiar or lifeless, there's still some good to be found here. Some of the action scenes are still pretty fun. Wonder Woman is still great regardless. And while some of the humor is a bit forced, some of it's still pretty good and it's not overdone. And there is still something to be said for watching guys like Batman, Wonder Woman and The Flash team up even if the result is a bit underwhelming.
Justice League smells of a film that had a troubled production, and the film definitely suffers for it. It's not a total failure, but it is definitely frustrating when you know it could've been so much better. While there's still some hope for the franchise yet with the threat of Darkseid (DC's Thanos) looming, one has to wonder if the franchise will last long enough to get to that point.